Transparent conductive coatings are used for many applications, especially in the fields of transparent electrophotography and dielectric film recording. Generally, current commercial product designs use either vacuum evaporated aluminum or sputtered indium doped tin oxide as transparent conductive coatings such as ground planes, but both processes are very expensive. Other conductive coatings have been obtained from conductivizing coating solutions containing various semiconductor materials in solution. The metal-containing semiconductor material is sometimes used in a complexed form with a complexing agent in a solvent solution. Generally, after the solution is coated onto a substrate, the complexing agent and solvent are driven or washed off, leaving the transparent, conductive coating.
Chemical Abstracts 95:195185f (1981) discloses the preparation of a solution derived conductive coating with cuprous iodide dispersed in poly(vinyl alcohol) and/or carboxy-methyl cellulose and/or maleic anhydride-methacrylic acid ester copolymer. These coatings are disadvantageous because they require use of an acetonitrile solvent for the cuprous iodide. The use of acetonitrile is undesirable for conductive coatings since acetonitrile is quite toxic at the levels indicated.
Another method for preparing solution derived transparent conductive coatings is disclosed in U.S. Pat. No. 3,245,833, issued to Trevoy. The conductive coatings described comprise a metal-containing semiconductor compound such as cuprous iodide dispersed in an insulating, film-forming binder. As complexing agents with cuprous iodide, Trevoy '833 mentions nitrogen-containing compounds such as volatile alkyl amines and volatile nitriles, such as acetonitrile.
The soluble species formed when cuprous iodide is dissolved in acetonitrile is thought to be tetrakis(acetonitrile)CuI which decomposes during the drying operation to yield finely divided particles of electrically conductive cuprous iodide dispersed throughout the coating layer.
The solution derived transparent conductive coatings shown in the Trevoy '833 patent have various disadvantages. Many of the complexing agents must be removed from the desolvated coatings by washing in order to achieve conductivity. Further, the use of aliphatic amines as solubilizing agents is complicated by deposition of sludge from the oxidatively unstable solutions obtained from aliphatic amines and cuprous iodide. Moreover, these conductivizing coating solutions provide coatings with a lack of uniformity and clarity on film substrates.
Other nitrogen-containing compounds are not useful as complexes with cuprous iodide for solution coating due to their insolubility. The analytical chemistry literature contains many examples of aromatic ligands, usually bidentates, useful in quantitative analyses to insolubilize copper salts, rather than solubilize them. Usually, the precipitation medium for these complexes has been water and the ligands, e.g. bipyridyl, biquinolyl, oxine, 1,10-phenanthroline, etc., have been found to form unstable colored solutions or precipitates. See generally, Organic Analytic Reactants, F. Welcher, D. Van Nostrand Co., N.Y., N.Y., 4th printing (1955).
Certain aromatic ligands containing nitrogen have been reported to form complexes with cuprous iodide which are soluble in organic solvents. Toluene solutions of substituted pyridine complexes with cuprous iodide were disclosed in a photoemission study by Kyle et al, J. Chem. Soc. Chem. Commun., pp. 714-715 (1989). Vogler et al, J. Am. Chem. Soc., 108:7211-7212 (1986), reported that cuprous iodide-pyridine complexes in benzene were unstable over time. Malik described stable complexes of cuprous iodide with pyridine and pyridine derivatives which are insoluble in water but soluble in organic solvents. J. Inorg. Nuc. Chem., 29:2106-2107 (1967). Such complexes, however, have heretofore not been suggested as being plausible or useful in the preparation of conductive coatings.
There is a need in the conductivizing art for a complexing agent with cuprous iodide which can be dissolved at useful concentrations in solvents with relatively low toxicity, relatively low boiling point and relatively low cost. Moreover, there is a need in the art for conductivizing coating solutions with good volatility, solubility and stability towards oxygen. There is a particular need in the fields of electrophotography and dielectric film recording for a conductivizing coating solution which can form an inexpensive, transparent, conductive coating.